Polycarbonate solid shaped articles, in particular polycarbonate solid sheets, are produced by extrusion or injection molding. Relatively large shaped articles, such as large-area motor vehicle glazing or sheets for roofing and other architectural uses, must be produced by extrusion because for technical reasons production by injection molding cannot take place, or at least cannot take place economically.
For extrusion, polycarbonate granules are fed to the extruder and melted in the plasticating system of the extruder. The melt of the plastic is forced through a sheet die and thereby shaped, brought into the desired final shape in the nip of a polishing calender and fixed in shape by reciprocal cooling on polishing stacks and in the ambient air. The polycarbonates of high melt viscosity used for the extrusion are conventionally processed at melt temperatures of 260 to 320° C., and the barrel temperatures of the plasticating barrel and the die temperatures are set accordingly.
Due to the process, the surfaces of the sheet produced are not completely flat but have irregularities. These lead to optical distortions. For various uses, for example glazing of motor vehicles, such optical distortions are undesirable, and for this reason a number of measures for improving the optical properties of extruded polycarbonate shaped articles have already been proposed.
To reduce reflection and optical imperfections of structured, weather-resistant sheets, EP 0 275 252 A proposes a polycarbonate composite sheet, on to the surface of which is applied a protective coating which comprises a methacrylate copolymer.
EP 0 114 290 A describes thermoplastic copolyester-carbonate molding compositions which comprise 1 to 15 wt. % of a branched polycarbonate and 15 to 99 wt. % of a copolyester-carbonate resin. This mixture is said to improve the processing properties and the resistance to hydrolysis as well as the impact strength. It is stated that the optical transparency of the polymer is not substantially impaired by the use of this molding composition.
EP 1 265 943 describes polycarbonate solid shaped articles with improved optical properties which are obtained from a branched polycarbonate of average molecular weight Mw 25,000–40,000. No other possibilities for achieving the aim are mentioned.
In the extrusion process already described, volatile constituents volatilize out of the melt of the plastic as soon as the melt emerges from the sheet die. In particular, these are UV absorbers, which in the case of a coextruded sheet are contained in a relatively high concentration in the coextruded thin layers on the sheet surfaces. These volatile constituents partly precipitate on the rolls of the polishing calender and form there a deposit which increases in the course of time and increasingly reduces the surface quality of the sheet produced.
EP 0 649 724 describes a process for the production of multi-layered sheets of plastic from branched polycarbonate having an average molecular weight Mw of 27,000–29,500, in which the volatilization of UV absorbers is reduced, so that extrusion can be carried out for a relatively long time without interruption, although the outer layer comprises 1–15 wt. % of a UV absorber. No other possibilities for reducing the volatilization of UV absorbers are mentioned.
The object of the present invention was to provide by extrusion, a transparent polycarbonate solid article, in particular polycarbonate solid sheets, having improved optical properties suitable for use in motor vehicles. The volatilization of volatile constituents should furthermore be reduced and the surface quality of the polycarbonate solid shaped articles therefore improved.